First, a frame structure and resource structure of a wireless communication system will be described with reference to FIGS. 1 and 2. FIG. 1 is a diagram showing an example of a frame structure of a wireless communication system. As shown in FIG. 1, one frame includes 10 subframes, and one subframe includes two slots. A time consumed for transmitting one subframe is referred to as a Transmission Time Interval (TTI). For example, one subframe may have a length of 1 ms and one slot may have a length of 0.5 ms.
One slot may include a plurality of orthogonal frequency division multiplexing (OFDM) symbols. The OFDM symbol may be called an SC-FDMA symbol or a symbol period.
One slot may include 7 or 6 OFDM symbols according to the length of a cyclic prefix (hereinafter, referred to as “CP”). In a long term evolution (hereinafter, referred to as “LTE”) system, there are a normal CP and an extended CP. In case of using the normal CP, one slot includes 7 OFDM symbols and, in case of using the extended CP, one slot includes 6 OFDM symbols. The extended CP is used when delay spread is large.
FIG. 2 is a diagram showing a resource structure of one downlink slot. FIG. 2 shows one slot including 7 OFDM symbols. A resource element (RE) is a resource region including one OFDM symbol and one subcarrier and a resource block (RB) is a resource region including a plurality of OFDM symbols and a plurality of subcarriers. For example, the RB may include 7 OFDM symbols in a time domain and 12 subcarriers in a frequency domain. The number of RBs included in one slot may be determined according to a downlink bandwidth.
An LTE-A system which is currently being standardized supports carrier aggregation. Carrier aggregation refers to technique of collecting a plurality of carriers so as to extend a bandwidth in order to increase a data rate.
Multicarrier refers to an overall frequency band used by a base station and a component carrier refers to an element carrier configuring multicarrier. That is, a plurality of component carriers configures multicarrier through carrier aggregation.
A user equipment (UE) of an LTE Rel-8 system completes random access on component carriers in which a single downlink and a single uplink are linked one-to-one, after completing contention-based random access. However, a UE of an LTE-A system using carrier aggregation may be allocated one or more uplink component carriers and timing advances of the allocated one or more uplink component carriers may differ. Accordingly, there is an ambiguity in timing advances of one or more component carriers.